Effects of three days of dry immersion on heart rate and blood pressure variabilities during head-up tilting in humans.

In this study we examined the effects of 3 days of dry immersion on heart rate variability (HRV) and blood pressure variability (BPV) in response to head-up tilting in 4 healthy young subjects aged 21 to 36 years. Resting value of the high-frequency (HF) power of BPV and HRV decreased, while the low-frequency (LF)/HF ratio of HRV and blood pressure increased after the dry immersion. The HF power of HRV decreased, while the LF/HF ratio of HRV and heart rate increased during head-up tilting before and after dry immersion. All powers of BPV and BP were unchanged during head-up tilting. The values of the decrease in the HF power of HRV and the increase in the LF/HF ratio during head-up tilting after dry immersion were larger than those before. These results suggest that 3 days of dry immersion altered the autonomic balance toward the sympathetic dominant at rest, and that cardiac function may play a crucial role in BP maintenance during head-up tilting compared with vasoconstrictor function.